In a process of manufacturing semiconductors, as an example of treatments for forming a film on a semiconductor wafer (hereinafter, referred to as a wafer), a treatment for laminating atomic layers or molecular layers of reaction products including a reasonable step for coverage (embedding property) of fine patterns or with a high compactness of a thin film is performed. Such a film forming treatment is performed by adsorbing a first gas onto a heated substrate, reacting a second gas with molecules of the first gas on the substrate, e.g., to nitride or oxidize the molecules, and repeating these processes a plurality of times.
In order to perform the film forming treatment on a plurality of wafers in clusters, a vertical heat treatment apparatus is used, and as an example thereof, there may be an example in which a silane-based gas and an ammonia gas are alternately supplied into a reaction chamber through gas nozzles different from each other to form silicon nitride films on the wafers. In addition, the ammonia gas is converted to plasma and supplied onto the wafers in order to reduce impurities in the reaction products by promoting the nitration of the silane-based molecules. Although a treatment is also known in which the ammonia gas is supplied without being converted to plasma, it is advantageous in that a heating temperature of the wafers can be lowered.
When such a film forming treatment is performed, thin films are formed on not only the wafers but also the interior of the reaction chamber. If a thickness of the film on the interior of the reaction chamber is increased, the film is peeled off to facilitate particle contamination. In this respect, if the thickness of the accumulated film exceeds a set value, the interior of the reaction chamber is cleaned with a cleaning gas. A fluorine-based gas is used as the cleaning gas, for example, when a silicon nitride film is to be formed. In addition, after the cleaning is performed, as the supply of the silane-based gas and the supply of the ammonia gas under plasma conditions are alternately repeated, a film forming treatment is performed so that a film having a predetermined thickness is formed on the inner wall of the reaction chamber. Such a treatment, which is referred to as pre-coating or the like, is performed in an initial film forming treatment on product wafers after the maintenance, in order to secure a stable film forming treatment by making a treatment atmosphere constant for a plurality of groups (or lots) of wafers.
However, if the film forming treatment is performed on the product wafers after the cleaning and the pre-coating is performed as described above, particle contamination occurs in the wafers of the first lot. On this account, the yield in the corresponding lot may become deteriorated.